Conventionally, a heat detector (thermal sensing device) which senses an occurrence of a fire by sensing heat generated by the fire is proposed. Such heat detector generally includes a heat detecting unit that senses heat in a monitoring area and a sensing-device main body that raises an alarm according to a state sensed by the heat detecting unit.
The heat detecting unit has a sensor unit which senses heat in a monitoring area and converts a sensed state into another state change. The sensor unit is formed of, for example, a diaphragm which changes shape according to an expansion of air caused by temperature rise, a thermistor which changes resistance according to the temperature, or a bimetal which changes shape in a predetermined direction according to the temperature (see, for example, Japanese Patent Application Laid-Open No. H05-266377).
In a heat detector utilizing a diaphragm, the diaphragm is deformed when an air in a chamber expands suddenly according to the temperature rise caused by a fire. The heat detector detects when a rate of temperature rise reaches a predetermined value or a higher value by detecting the presence/absence of the deformation of the diaphragm. Then, the heat detector determines that a fire occurs in a monitoring area and outputs an alarm signal.
In a heat detector utilizing a thermistor, electric resistance of the thermistor changes according to the temperature rise caused by a fire. The heat detector detects changes in the electric resistance. When the changes in the electric resistance indicates that a rate of temperature rise is equal to or higher than a predetermined rate, the heat detector determines that a fire occurs in a monitoring area and outputs an alarm signal. FIG. 19 is a front view of a conventional thermistor-type heat detector, and FIG. 20 is a sectional view along A-A of the heat detector of FIG. 19. A heat detector 110 schematically includes a sensing-device main body 111 and a thermistor 112 which is arranged so as to protrude from one side surface of the sensing-device main body 111. The thermistor 112 is arranged like a projection so that the thermistor 112 is placed as far from the sensing-device main body 111 as possible for the prevention of thermal conduction between the sensing-device main body 111 and the thermistor 112, and at the same time so that an air flow from outside the sensing-device main body 111 hits the thermistor 112 as directly as possible for an enhancement of thermal responsiveness of the thermistor 112. Additionally, a thermistor guide 113 is arranged around the thermistor 112 for a protection of the vertically-arranged thermistor 112 from outside.
A heat detector utilizing bimetal utilizes a characteristic of bimetal, which is deformed in a predetermined direction according to the temperature; when the temperature reaches a predetermined temperature or higher and the bimetal undergoes large deformation to close an electric contact, the heat detector determines that a fire occurs in a monitoring area and outputs an alarm signal (see, for example, Japanese Utility Model Application Laid-Open No. H6-30891).